This invention relates to guns which use a charge of compressed air to fire a pellet.
Air guns have a wide following because laws limiting their use are not as restrictive as for powder guns, and air guns are relatively inexpensive to shoot. Air gun shooting is an Olympic sport, and hunting with an air gun removes much of the danger inherent with powder guns while retaining and enhancing the challenge.
Air guns fall into three major groups:
1. Pump guns: These guns use one or more strokes from a pumping device to store a charge of compressed air in a firing chamber. The required effort to charge the gun increases with each pump as the stored pressure builds. The power of the gun depends on the strength of the shooter because the relatively low mechanical advantage of the pumping mechanism. Most of these guns completely expel the air charge when fired. On firing, the pellet is initially exposed to the full pressure of the compressed air, but the available pressure falls rapidly as the pellet accelerates down the gun barrel. These guns usually have moderate power, driving a pellet at about 500 feet per second. U.S. Pat. No. 4,572,152 to Olofsson, et al., discloses an air gun which uses a floating piston to store compressed air in an auxiliary chamber. The purpose of the floating piston is to augment firing pressure by moving to displace air in the firing chamber when the gun is fired. However, with the gun disclosed in the Olofsson, et al. patent, the compressed air stored in the auxiliary chamber is limited to that provided by one stroke of the pump, and the pressure in the auxiliary chamber can never be greater than the pressure in the firing chamber.
2. Spring guns: These guns use a single stroke of a lever to compress a steel spring. On firing, the spring drives a relatively heavy piston that causes a rapid increase in air pressure within a firing chamber. The firing chamber is directly connected to the gun barrel. The pellet is held in the gun barrel by a seal until the air pressure in the chamber reaches an optimum point. When this happens, the air pressure overcomes the holding ability of the seal and drives the pellet down the barrel. The piston also continues to displace air in the firing chamber, thereby helping to maintain pressure on the pellet. This method has replaced multi-stroke pumping as the most common air gun mechanism. Only one stroke of the lever accomplishes the entire cocking procedure. Thus, a spring gun usually takes less time to place into action than a multi-stroke gun. By maintaining a more constant force on the pellet as it travels down the barrel, the imparted energy may be twice that available with a conventional pneumatic multi-pump gun. However, the drawback of a spring gun is that only one stroke of the lever is available to compress the spring. The most powerful spring guns require strength beyond the limit of many people. Moreover, the spring imposes a practical limit on the amount of energy that can be stored. At least one model has replaced the steel spring with a compressed air xe2x80x9cspring.xe2x80x9d The compressed air in the xe2x80x9cspringxe2x80x9d is not expended but is re-compressed with the gun""s lever. The air spring can store more energy in a smaller space, but considerable work must be expended by the shooter.
3. Pre-charged guns: These guns use a gas charge that is pre-packaged and inserted into the gun with little expenditure of energy by the user. The most common guns of this type use a small container of liquid carbon dioxide to power the gun. Each firing of the gun uses a portion of the stored liquid, which rapidly vaporizes on firing. A method gaining popularity transfers compressed air from a storage bottle into a relatively large storage vessel attached to the gun. For example, air from a diver""s scuba tank or similar storage vessel is transferred into the storage vessel on the gun through a high-pressure hose and clamp assembly. The gun gets multiple shots from charges provided by the air in the storage vessel, but the accuracy of the gun diminishes with the loss of available pressure until the storage vessel is refilled. Some carbon dioxide (CO2) guns use small canisters available at hardware stores. These guns are moderately powerful, but also suffer from accuracy problems with the loss of pressure in the canister. Guns which use compressed air from large detached tanks can store more energy and suffer less in accuracy lost between shots. However, the detached tank (such as a scuba tank) is heavy and cumbersome.
In summary, multiple-pump air guns are limited by the strength of the user, and the initial strokes are time consuming for the amount of useful energy transferred to the storage chamber. Spring guns use one quick pull of a lever and achieve efficiency with the available energy, but are limited by the strength of the individual loading the gun. Guns which use a pre-charged vessel of compressed gas must have the vessel in close proximity to the gun, and cannot rely on precision repeat performance with each shot.
Maximum muzzle energy for the three types of guns is about 11.5 foot-pounds for the best multi-pump guns, about 25 foot-pounds for the best spring guns, and about 30 foot-pounds for the best pre-charged gun using air from a scuba tank.
Convenient power is the goal of air guns. With more power the pellet trajectory is flatter, accuracy is enhanced, and more energy is delivered at the point of impact.
This invention provides an air gun which stores and imparts increased shooting power without requiring the shooter to be of more than average strength. The gun uses a unique pumping action with a large mechanical advantage to store energy and efficiently transfer stored energy to the pellet to achieve muzzle energy in excess of 40 foot-pounds.
The air gun of this invention uses an improved air pump which includes a pump cylinder and a pump piston mounted to reciprocate within the cylinder. The pump cylinder and a piston rod connected to the piston are each connected to the barrel of the gun to pivot about separate respective longitudinally spaced axes, which are transverse to the longitudinal axis of the barrel. As the pump cylinder and piston rod are moved back and forth around their respective the pivot points, the cylinder and piston reciprocate relative to each other to pump air into an inlet of a high pressure housing carried by the pump cylinder. A firing conduit connected to the high pressure housing releasably connects an outlet of the high pressure housing to the breech end of a gun barrel when the pump cylinder is moved to be parallel with the barrel. A trigger-responsive firing valve in the firing conduit releases air from the high pressure housing into the breech end of the barrel to fire a pellet from the gun.
In a preferred embodiment, the piston rod is secured at one end to the piston, and at the other end to a first pivot point on the gun barrel. An elongated drive link is secured at one end to a pivot point on the cylinder, and at the other end to a second pivot point on the gun barrel, so that as the cylinder and piston are moved back and forth about the first and second pivot points, the piston reciprocates in the cylinder to force air through a check valve and into the high pressure housing. The length of the drive link and the longitudinal spacing between the first and second pivot points are set so when the pump cylinder is moved to be substantially parallel to the barrel, the piston contacts the check valve which admits air into the high pressure housing so a maximum amount of compressed air is transferred to the high pressure housing with each compression stroke of the pump. The first pivot point is located to the rear of the second pivot point and is spaced slightly farther from the longitudinal axis of the gun barrel so when the pump cylinder is moved toward the gun barrel to a xe2x80x9cdead centerxe2x80x9d position, which places the longitudinal axis of the piston rod and piston substantially in alignment with the first and second pivot points, the piston contacts the check valve with maximum force. At this point, the pump cylinder extends rearwardly and away from the gun barrel to leave ample space for gripping the rear end of the cylinder to actuate the pump. Further movement of the pump cylinder toward the gun barrel carries the piston rod and piston slightly past the xe2x80x9cdead centerxe2x80x9d position. The elasticity inherent in the gun and pump components accommodates movement of the pump cylinder back and forth through the xe2x80x9cdead centerxe2x80x9d position, which acts as a moderate detent to hold the pump cylinder snugly against the barrel when the gun is to be prepared for firing.
In a further preferred embodiment of the invention, a floating differential piston is disposed to move longitudinally within the high pressure housing and divide the housing into a storage chamber adjacent the housing inlet and a firing or discharge chamber adjacent the housing outlet. A pressure relief valve in a pressure relief conduit extending through the floating differential piston from the storage chamber to the firing chamber maintains a higher pressure in the storage chamber than in the firing chamber. Preferably, the pressure relief valve is adjustable. The diameter of the end of the floating differential piston adjacent the storage chamber is smaller than the diameter of the end of the piston adjacent the firing chamber. A first sliding seal is provided between the interior of the high pressure housing and the smaller end of the piston. A second sliding seal between the housing interior and the larger end of the piston seals a larger cross-sectional area of the housing than the first seal. When air pressure in the storage chamber exceeds the differential pressure set by the pressure relief valve in the pressure relief conduit, air flows through the conduit from the storage chamber and into the firing chamber until the pressure in the firing chamber reaches a value which permits the pressure relief valve to close. As the pressure in the two chambers increases, the larger cross-sectional area of the firing chamber sealed by the larger end of the floating differential piston causes the piston to move toward the inlet end of the housing, thereby reducing the volume of air in the storage chamber and increasing the volume of air stored in the firing chamber until the forces acting on each end of the piston are balanced. Additional pumping stores more compressed air in the storage and firing chambers until the desired firing pressure is reached. When the firing valve in the housing outlet releases compressed air from the firing chamber in response to pulling the trigger on the gun, compressed air in the storage chamber expands and drives the floating differential piston toward the housing outlet as compressed air in the firing chamber enters the barrel breech to drive a pellet out the barrel. Thus the compressed air in the storage chamber expands and drives the floating differential piston toward the outlet of the firing chamber to maintain a more uniform pressure on the pellet as it is fired. The pressure relief valve in the floating piston tends to open momentarily when the firing of the gun suddenly drops the pressure in the firing chamber. However, loss of compressed air from the storage chamber is minimized because the flow path for air from the storage chamber to the firing chamber is so restricted, that only a small amount of air is lost from the storage chamber before the pressure relief valve closes. The lost air is quickly replaced when the pump is operated for the next shot. Preferably, the mass of the floating differential piston is as low as practical, and a mechanical compression spring also urges the floating piston toward the firing valve to further minimize loss of air from the storage chamber when the gun is fired.
The gun of this invention supplies such a large mass of high-velocity compressed air behind the pellet as the pellet leaves the barrel, there is a tendency for the air to overrun the pellet and cause it to precess or tumble, which would destroy accuracy. To avoid this problem, the muzzle end of the rifle barrel includes at least one lateral opening through the barrel to vent some air under pressure before the pellet leaves the barrel.